U.S. Pat. No. 5,922,149 discloses a method for making steel wires and shaped wires used for enforcement of flexible tube. A shaped wire is produced by rolling or drawing steel consisting of 0.05-0.5% C, 0.4-1.5% Mn, 0-2.5% Cr, 0.1-0.6% Si, 0-1% Mo, no more than 0.25% Ni, and no more than 0.02% S and P, and a first heat treatment is performed on the shaped wire, including at least one step of quenching under predetermined conditions to achieve an HRC hardness of at least 32, a predominately martensitic and bainitic steel structure and a small amount of ferrite. The quenching step comprises passing said steel wire through an austenitizing furnace at a temperature that is greater than point Ac3 of the steel. The shaped wire has a breaking point Rm which does not exceed 900 MPa after the thermal treatment.
International patent application No. 2011/151532 discloses a profiled wire of low-alloy carbon steel intended for use as flexible tube component. The steel wire has following composition: carbon between 0.75% and 0.95%, manganese between 0.30% and 0.85%, chromium less than 0.4%, vanadium less than 0.16%, silicon between 0.15% and 1.40. This steel wire is manufactured by first hot rolling elongated element rod in its austenitic domain followed by cooling down to room temperature. The profiled wire is obtained by first subjecting the wire rod to a thermo-mechanical treatment by two consecutive and ordered phases, namely, an isothermal tempering to confer on the wire rod a homogeneous perlitic microstructure, followed by a cold mechanical transformation operation with an overall work hardening rate comprised between 50 and 80% max, to give it its final shape. The obtained profiled wire is then subjected to a heat treatment at a temperature from 410 to 710° C. giving it the desired final mechanical characteristics. In the patent application, the micro-structure to be created by the isothermal tempering is pearlite to make the steel withstand the deformations applied by drawing and/or rolling.
Carbon steels in the cold-shaped raw states that have a ferrite-pearlite structure and considerably high mechanical strength and hardness values are commonly used. It has been found, nevertheless, that increasing mechanical strength beyond certain limits causes such steels to have inadequate ductility, taking into account e.g. the pre-shaping and bending operations that have to be carried out with spring wire, and reinforcement operations that are needed for reinforcement wire. International patent application WO2013041541 has disclosed a specific heat treatment on a steel wire with a particular steel composition. Thus obtained steel wire has a metallurgical structure with certain volume of retained austenite and high elongation at fracture. A lot of efforts have been done on the steel wires to further improve tensile strength and simultaneously have acceptable or desirable ductility.